We have shown that a 4.9kb replication defective recombinant murine leukemia virus (MuLV), present in the LP-BM5 murine leukemia virus complex, is the proximal cause of a lymphoproliferative, immunodeficiency syndrome, murine AIDS (MAIDS). The defective virus (BM5def) was molecularly cloned and sequenced to reveal that the virus has a functional LTR and the gag gene is intact. Within the gag gene only the p15 and p12 are highly divergent from other MuLV. In studies of the role of gag protein(s) in pathogenicity we have found that although bacterially expressed p12 gag induced high levels of antibodies in both susceptible and resistant mice there was no induction of a protective immune response. In order to determine the consequences of constitutive expression we prepared 2 constructs of BM5 def gag for use in establishing transgenic mice. One utilized SV40 promoter and immunoglobulin (Ig) heavy chain enhancer to direct the gag gene expression in B cells and although 1/8 founder mice expressed the transgene, it did not transmit through the germline. The second construct utilizes MHC class II (Ea) promoter, to allow expression in B cells, macrophages and dendritic cells. Selection of transgene positive mice from the Ea promoter construct is in progress. We also developed two gag gene expression vectors (one with beta actin promoter and the other with CMV promoter) and used them to transfect B cell lineage tumor cell lines. None of the surface-positive transfectants were able to stimulate the proliferation of normal spleen cells. We are continuing such studies with other promoters. To explore the effects on pathogenicity of alterations in the structure of various segments of the BM5def genome, we have constructed a biologically active genomes containing LTR enhancer elements from a highly leukemogenic ecotropic virus and are preparing chimeric genome containing non-pathogenic ecotropic elements. We used a 100bp probe derived from the p12 gag region of BM5def to demonstrate its presence in the tissues of infected mice by restriction endonuclease digestion and southern blotting. In some instances, especially when a defective virus containing a bacterial neomycin resistance gene was used to induce disease, we were unable to detect the defective viral genome in the involved tissues by similar techniques. We thus developed primers and probes to detect defective genomes by PCR reactions.